Wideband Code Division Multiple Access (WCDMA) communication, as used in third generation (3G) mobile cellular communications air interface technologies, is an ‘interference limited’ technology from a data throughput perspective. CDMA technology utilizes orthogonal variable spreading factor (OVSF) codes combined with pseudo noise (Pn) codes to differentiate between multiple wireless subscriber communication units (referred to as user equipment (UE) in 3G terminology) that are utilizing the same spectrum at the same time. Often, up to ninety six UEs are simultaneously supported in call mode for a specific base station (referred to as Node-B in 3G terminology). Thus, a major factor in the design of cellular communication systems, is the ability to differentiate between different communications (or communication units).
Conventional antenna arrays, comprising multiple antenna elements and used with existing Node-B equipment in most 3G installations, utilize a fixed 65° beam pattern. Outside of the main lobe of the antenna beam the signals are spatially filtered and significantly attenuated. Conventional network planning and passive antenna array solutions process all incoming signals with a common fixed beam pattern. Such receive processing, based on signals received within the geographic area identified by the antenna beam main lobe, referred to as the RF footprint, tends to dictate a corresponding common beam pattern for transmitter operation. Thus, an identical radio frequency (RF) footprint is used for both receive (Rx) and transmit (Tx) operation.
Receive beam-forming using antenna arrays depends on the ability to constructively add incident signals on each of the antenna elements in a way that coherently adds those from the desired direction. Thus, incident signals that are not from the desired direction will be incoherently added, and thus will not experience the same processing gain. The term ‘coherency’ implies that the signals will have substantially the same phase angle. In addition, thermal noise from multiple sources also exhibits incoherent properties, and thus when added the signals from multiple sources do not experience the same processing gain as a coherent desired signal. Conversely in transmit active antenna arrays the signals are coherently combined within the intended beam pattern as electromagnetic (EM) signals in the ‘air’ so that they arrive coherently at the mobile station (MS) (e.g. UE) receiver.
Due to the number of concurrent communications that a Node-B is required to support, a number of techniques have been developed to assist in ensuring that a radio link performance adheres to adequate service levels, such as antenna diversity (where beam patterns are used to focus on (and separately process) communications to a particular sector of a cell) and polarization diversity (where different polarizations of antenna are used in order to maximize a received signal level from different communications units.
HSPA+, also known as Evolved High-Speed Packet Access is a wireless broadband standard defined in 3GPP release 7. HSPA+ provides HSPA data rates up to 56 Mbit/s on the downlink and 22 Mbit/s on the uplink with multiple input and multiple output (MIMO) technologies and higher order modulation (64QAM). Recent trials in HSPA+ networks have uncovered a problem with capacity and coverage issues with single antenna UE devices, The intention of HSPA+ is that it should be backward compatible to all network UEs including those supporting just HSPA and Release 99 versions of the 3G standard. HSPA+ introduces and utilizes transmit diversity on the Node-B network element. MIMO transmission on HSPA+ intends to use polarization diversity for transmission of the second carrier on the same frequency as the first. Network operators and 3GPP standards intend to use a common pilot channel (CPICH) on one of the polarization transmissions and no CPICH on the other. CPICH is used by UE devices in the rake receiver for both the channel equalisation and rake receiver channel estimator. In the absence of a CPICH, for example if it is not transmitted from the node-B, alternate equalisation and rake receiver channel estimator techniques may be employed. Usually an algorithm such as a minimum mean square error (MMSE) algorithm is used to estimate the weights and delays of the Rake receiver in WCDMA based receptions without the CPICH being present.
Single antenna legacy 3G UE devices typically use just one equalisation method at any one time. If for example the CPICH is used, then the MMSE mode is disabled and vice-versa. A network in HSPA+, transmitting with both CPICH and non-CPICH modes on orthogonal polarizations, implies that an untracked and uncorrelated transmission is used to that is not capable of being equalized out with just one equalisation method. Thus, a disastrous impact on single antenna UE receiver performance manifests itself. HSPA+ enabled devices with receive diversity are capable of increased data rate throughput by exploiting the second transmit diversity path on the Node-B.
Orthogonal LHCP (Left Hand Circularly Polarized) and RHCP (Right Hand Circularly Polarized) antennas for MIMO (Multiple Input Multiple Output) transmission in network trials has proved to be successful in minimising this problem with single antenna UE devices. This implies that an antenna for the node B must be capable of concurrent transmission in LHCP and RHCP, whilst reception using a XPOL (cross polarized) antenna is still preferred, given that UE devices have been predominantly designed using LP (Linear Polarized) antennas.
In the examples herein described, an antenna element is a radiative structure whose purpose is to convert electro-magnetic (EM) signals to electrical signals, or vice versa, in which a singular element has a fixed radiation pattern. The term ‘radiative elements’ described herein refers to elements capable of radiating an electromagnetic signal. Furthermore, the term ‘radiative elements’ described herein also encompasses structures capable of absorbing EM radiation and converting to electrical signals. These elements, constructed as an array can be configured to have various radiation patterns by manipulation of electrical signals coupled to the elements. Thus, the ability to alter the radiative beam shape may be achieved.
For completeness, it is worth clarifying the Antenna Reciprocity Theorem, which in classical treatises on electromagnetic fields and antennas is usually formulated as follows:
Given two antennas ‘A’ and ‘B’ placed at some distance apart, each of them may be operated either as a transmitting antenna or as a receiving antenna. Suppose that antenna ‘B’ is kept intact, whilst the performance of antenna ‘A’ as a transmitter is modified. A consequence of this is that, for a fixed amount of input power, the signal received by antenna ‘B’ changes by a factor ‘F’ due to the change imposed on antenna ‘A’. Then the same modification changes also the performance of antenna ‘A’ as a receiver and does so by the same factor ‘F’. The theorem follows from certain symmetries of Maxwell equations and its validity is easily verified experimentally and has been widely published. Hence, the radiation pattern induced by a transmitter operably coupled to an antenna with same carrier frequency as a receiver has identical azimuthal angular link loss. Thus, the term radiative and ‘radiative beam pattern’ hereinafter may also be applied to a receiver.
Referring now to FIG. 1, examples of known electromagnetic waveforms are illustrated. A first diagram 100 illustrates a linear polarized field from an antenna and a second diagram 150 illustrates a circular polarized field. The polarization of an antenna is the orientation of the electric fields (E-plane) 110 of the radio wave with respect to the Earth's surface and is largely determined by the physical structure of the antenna and by its orientation. The magnetic field (H-plane) 120 is always perpendicular to the E-plane 110. The E-plane 110 and H-plane 120 are respectively illustrated as propagating in the directions 105, 115. In contrast, circular polarized (CP) antennas as illustrated in the second diagram 150 have a rotating E-plane 160 in a propagation direction 155, in contrast to the linear polarized (LP) antennas having a fixed E-plane.
Circular polarization is the polarization of electromagnetic radiation, such that the tip of the electric field vector describes a circle in any fixed plane intersecting, and normal to, the direction of propagation. However, in practical systems there will be minor deviations from this perfect angular electric field vector that describes a circle. For the purposes of the description hereinafter described an E-Field vector that is substantially close to that of a circle is considered to be a circularly polarized field.
Elliptical polarization is the polarization of electromagnetic radiation, such that the tip of the electric field vector describes an ellipse in any fixed plane intersecting, and normal to, the direction of propagation. Elliptical polarized fields can be configured as circularly polarized fields, and can be rotated polarized fields in a clockwise or counter clockwise direction as the field propagates; e.g. forming right hand elliptical polarization and left hand elliptical polarization respectively. An elliptically radiated field will have substantially changed magnitude for 90° change in angular vector.
Cross-polarization (XPOL) antennas are also often used, particularly in cellular infrastructure deployments. XPOL antenna technology utilizes pairs of two LP antenna elements that are orientated substantially 90° with respect to each other, often referred to as being ‘orthogonal’ to each other, usually at +45° and −45° polarization. These pairs are often elements in an array, and thus can be arranged such that a desired propagation beam shape is developed. To date, deployed cellular infrastructure transmit polarization orientation predominantly only uses one of the polarization types whereas receive functionality is performed in both polarizations, with separate and independent processing of the two XPOL receive paths being employed. These XPOL antennas can be of patch construction (PCB) or of Dipole (Wire) construction.
Increasingly single mobile device and infrastructure equipment has to be capable of processing data for communication over more than one air interface access method. For example, the interfaces may include 3G, satellite, digital broadcast, etc. Many of these air interface methods may use different polarization techniques. For example DVB-SH s-band satellite broadcast services use CP, whereas cellular infrastructure use, as a general rule, +45° LP polarization transmission.
A known problem in using LP transmissions is that the polarization of the transmitted signal antenna and the receiving signal antenna need to have the angle of polarization exactly the same for reception of the strongest signal. For example a signal transmitted on a vertically polarized (VP) antenna and received on an antenna with horizontal polarized (HP) may have 10's of dB difference in received power compared to a matched VP antenna. Mobile handset antennas are generally LP, though increasingly through means of diversity reception paths a second polarization diversity LP antenna is utilized, orthogonally polarized to the first.
Furthermore, using CP the orientation of the receiving device is less critical to reception performance. However, an undesirable loss of around 3 dB in signal level, over the peak signal achievable with a matched LP antenna, would generally be expected of an equivalent CP antenna if receiving LP polarized wireless signals where each antenna has similar antenna gain.
WO/2000/001033 describes a portable adaptive polarization scanning satellite antenna system that performs receive only functionality using an analogue adjustment of RF phase shifters, in order to rotate the phase of the received signals at XPOL antenna elements. The system disclosed in WO/2000/001033 performs receive-only functionality of one reception type at any single instant in time. The single reception type at any single instant in time includes configuring VP and HP antennas to receive independently LHCP or RHCP signals. Thereafter, phase shifting XPOL elements is performed to better match incoming signal polarization.
U.S. Pat. No. 5,068,668 describes an adaptive polarization combining system that attempts to adapt an angle of an incoming signal phase of either of two polarized antennae, such that two incoming signals can be maximally combined, i.e. in phase. U.S. Pat. No. 5,068,668 describes a system that uses two orthogonally placed antennae of either CP or LP polarization. Again, the system disclosed in U.S. Pat. No. 5,068,668 performs receive-only functionality of a single signal reception path of one reception type at any single instant in time. Furthermore, U.S. Pat. No. 5,068,668 does not change the polarization type of the signals processed through the antenna, but provides a scheme to adaptively add them to maximise received signal strength.
U.S. Pat. No. 4,737,793 describes a radio frequency antenna with controllably variable dual orthogonal polarization of two signals (of common polarization type but orthogonal, such as LHCP and RHCP) to be applied to the antenna. U.S. Pat. No. 4,737,793 proposes a microstrip-based XPOL antenna with radio frequency-based control of both amplitude and phase of input signals that requires a power divider at the antenna inputs for splitting and phase adjusting the signal to be transmitted. Again, as the phase shifters are connected directly to the antenna only one polarization type is capable of reception and or transmission at any single instant in time in the system proposed in U.S. Pat. No. 4,737,793. Furthermore, the use of excessive processing on the signals at the antenna is undesirable, as the losses induced would be excessive and cause noise figure degradation of the receiver performance and an unacceptable loss on the PA output for transmission.
Thus, there is currently no known method of configuring an antenna polarization type using the same antenna element and supporting simultaneous reception modes of signals exhibiting different polarization types. Furthermore, the few known techniques in configuring an antenna polarization type utilize bulky radio frequency components, which add to the complexity, size and cost.
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of supporting antenna array technology in a wireless communication network would be advantageous.